Enable "shadow tracking". This means a special dict representation is used
together with `multidicts`_. This dict representation is used only for instance
dictionaries. The instance dictionary tracks whether an instance attribute
shadows an attribute of its class. This makes method calls slightly faster in
the following way: When calling a method the first thing that is checked is the
class dictionary to find descriptors. Usually, when a method is found, the
instance dictionary is then checked for instance attributes shadowing the class
attribute. If we know that there is no shadowing (since our instance dict tells
us that) we can save this lookup on the instance dictionary.

.. _`multidicts`: objspace.std.withmultidict.html
